Standards limiting SO.sub.2 emissions from flue gases produced by burning high sulfur content crude oils (e.g., 0.5-4.0%) in steam generators and other applications have been established and economical processes for removing SO.sub.2 are needed. One of the more economical methods of meeting these standards comprises scrubbing the gas in a gas-liquid scrubber using an alkaline pH adjusted solution. Sodium hydroxide is typically used to maintain the alkaline concentration of the solution, with make-up water from available sources being needed to replace evaporative and blowdown losses. Removal efficiencies vary, depending upon the scrubber design, sulfur content of fuel, pH/concentration of recycle liquor, etc., but typically run 85-98% with the pH of the recycle liquor usually being in the 6.0-7.0 range.
However, utilization of available water sources to supply make-up water usually presents scaling problems, the high calcium content producing a scale in the form of CaSO.sub.4 (the most difficult type of scale to remove), CaSO.sub.3 (which can be removed by acid washing), and CaCO.sub.3 (which occurs if the water pH is too high (e.g., above about 8.0). These scales build up in the scrubber requiring periodic shutdown and cleaning.
A solution most commonly used by oil companies to prevent such scaling problems is the use of a water softener to supply a low-hardness water to the steam boiler and meet the water-supply demands of the scrubber. These procedures are, however, costly as additional equipment, maintenance, etc., is required thereby.
An alternative solution is the addition of scale inhibitors to the make-up water that supplies the scrubbers. However, this procedure suffers the disadvantages in that additional feed equipment must be set up and that the feed must be monitored. Additionally, many of these inhibitors, e.g., calcium chelating agents, are of very limited solubility in concentrated sodium hydroxide solutions generally used to maintain an alkaline pH in the scrubber, thus requiring a two-step treatment or the uneconomical use of very dilute caustic solutions.
It would therefore be desirable to provide an effective scale inhibitor having good solubility in concentrated solutions of sodium hydroxide, thereby allowing a one-step addition of caustic and scale inhibitor to the scrubber, as well as the economical shipment of concentrated inhibitor-containing caustic solutions.